The invention relates to a process for co-production of paraxylene and ethylbenzene from an aromatic hydrocarbon feedstock that contains isomers with 8 carbon atoms.
The invention applies particularly to the synthesis of very pure paraxylene for producing a petrochemical intermediate product, terephthalic acid.
The prior art is illustrated by Patent Application FR-A-2 773 149.
The production and separation of paraxylene are carried out in industrial practice by arranging the following in a loop:
a process for separation of the paraxylene by adsorption (U.S. Pat. No. 2,985,589, U.S. Pat. No. 3,626,020), whose effluents are paraxylene, on the one hand, and an aromatic C8 fraction that is substantially free of paraxylene, on the other hand. Crystallization can be combined with the adsorption stage to obtain paraxylene that is more pure (U.S. Pat. No. 5,284,992, U.S. Pat. No. 5,401,476);
a process for isomerizing the aromatic C8 fraction that treats the second of the two effluents of the separation unit and produces an isomerate that contains paraxylene. This isomerate is recycled to the feedstock stream that feeds the paraxylene separation unit.
There are two classes of processes for isomerization of aromatic compounds with eight carbon atoms: the first class is known under the name xe2x80x9cconverting isomerizationxe2x80x9d because ethylbenzene is in part converted into xylenes, which are in proportions that are close to those of the thermodynamic equilibrium. The catalysts that are used in the converting isomerization steps are bifunctional. A catalyst with a zeolite base ensures the conversion of orthoxylenes and metaxylenes into paraxylene by migration of the methyl groups. As a result, at the temperature in question, thermodynamic equilibrium is nearly reached among the three xylenes: at 400xc2x0 C., typically, orthoxylene 24%, metaxylene 52%, and paraxylene 24%. Dispersed platinum ensures, in the presence of hydrogen, a hydrogenating-dehydrogenating function that makes it possible to convert the ethylbenzene into a mixture of xylenes. Hydrogen is necessary for producing the naphthenic intermediate products that yield xylenes after dehydrogenation.
The operating conditions of the isomerization are often dictated by the conversion of ethylbenzene: temperature and partial hydrogen pressure. The intermediate reactions for conversion of the ethylbenzene lead to the presence of a significant proportion of naphthenes in the loop. The applied temperature is increased to ensure the desired paraxylene production. Taking into account the compositions of the fresh feedstock and of the isomerate, it is necessary to treat the flow of fresh feedstock 3 to 5 times in the separation unit to produce about 0.85 times the flow of fresh feedstock in the form of paraxylene. The 5 to 10% of fresh feedstock that is not converted into paraxylene is found in the form of cracking and transalkylation products.
The second class of isomerization processes is known under the name of dealkylating isomerization.
In this type of isomerization, ethylbenzene is converted into benzene and ethylene on catalysts with a ZSM5 zeolite base, while the xylenes are brought into thermodynamic equilibrium. Hydrogen is also needed here to hydrogenate into ethane the ethylene that is formed (to prevent realkylation) and to prevent the coking of the catalyst. The H2/HC ratio, however, is considerably lower than that found in converting isomerization. In this case, co-production in the separation-isomerization loop of paraxylene (about 78%) and benzene (15%), with 7% of various losses, is ensured. Here again, the temperature conditions are still dictated by the fact that it is necessary to dealkylate the ethylbenzene.
In contrast, in industrial practice, ethylbenzene is the reaction intermediate product that makes it possible to obtain styrene by dehydrogenation. Ethylbenzene is always produced by alkylation of benzene with ethylene. These alkylation units require a reactor with considerable recycling to be able to control the exothermicity of the reaction and, moreover, a number of distillations finally to separate gases, benzene, ethylbenzene, and di-, tri- and tetraethylbenzene.
Molecular sieves that can separate ethylbenzene from xylenes have been described effectively (Patents U.S. Pat. No. 4,497,972, U.S. Pat. No. 5,453,560). Despite the respectable separation performance levels of these sieves, to our knowledge no commercial unit for separation of ethylbenzene in a simulated moving bed has been built to date.
The prior art actually has always regarded the production of ethylbenzene as an isolated problem. If it is considered that the aromatic C8 feedstocks from which ethylbenzene is to be extracted contain at most 16%, a process for separating ethylbenzene, such as, for example EBEX(R), is more expensive than a unit for alkylating benzene. This way of looking at things has quite often been reinforced by the fact that the locations where paraxylene and orthoxylene, on the one hand, and those of styrene, on the other, are produced are generally geographically different: actually, the xylene production line is most often integrated into a refinery to keep from having to transport the aromatic C8 fraction. In some cases, however, it is integrated into a plant for producing terephthalic acid or methyl terephthalate. By contrast, the ethylbenzene production line is generally integrated into a plant for producing styrene and polystyrene.
An object of the invention is to eliminate the drawbacks that are mentioned above.
Another object is the co-production of ethylbenzene and paraxylene.
Another object is the improvement of the performance levels of the aromatic loop with the use of an isomerization catalyst that comprises an EUO-structural-type zeolite that preferably contains the EU-1 zeolite.
Another object relates to the analogous production of essentially pure metaxylene and orthoxylene when the intent is not to maximize the production of paraxylene.
It has been observed that by combining a first adsorption step, from which an ethylbenzene-rich raffinate was drawn off, with a second adsorption step of this raffinate, from which a fraction that is very low in ethylbenzene and that contains orthoxylene and metaxylene that have been subjected to isomerization under favorable conditions would be drawn off, very good results under very economical conditions were obtained.
More specifically, the invention relates to a process for co-production of paraxylene and ethylbenzene from an aromatic hydrocarbon feedstock (1) that contains isomers with 8 carbon atoms, in which in the presence of a first desorbent (6a), said feedstock is brought into contact with a zeolitic adsorbent in a first adsorption unit (2) in a simulated moving bed; a first paraxylene-rich fraction (4) and a second fraction (R1) (3) that is low in paraxylene and high in ethylbenzene are drawn off; said second fraction (R1) is brought into contact with a second suitable adsorbent in a second adsorption unit (8) in a simulated moving bed in the presence of a second desorbent (12a); a third fraction (9) that comprises essentially pure ethylbenzene and a fourth orthoxylene-rich and metaxylene-rich fraction (10) that essentially no longer contains ethylbenzene are recovered; at least a portion of the fourth fraction is isomerized in an isomerization zone (14) in the presence of a catalyst that comprises an EUO-structural-type zeolite; an isomerate (16) is collected, and it is recycled in first adsorption unit (2).
The two adsorption units are generally used according to the simulated moving-bed technique.
According to an advantageous characteristic of the process, it is possible to determine an additional chromatographic zone in first adsorption unit (2) by using five zones instead of four, for example. Said zone is introduced downstream from the draw-off of second fraction (R1) so as to collect the second fraction with a minimal first desorbent content, and downstream from said chromatographic zone, another fraction R2 (3a) is drawn off that is low in paraxylene and high in orthoxylene and metaxylene but that essentially no longer contains ethylbenzene, and at least a portion of said fraction (R2) is isomerized in isomerization zone (14).
The invention offers the following advantages:
The productivity of the separation process of paraxylene in the first adsorption unit is improved because the recycling of the isomerate that contains very little ethylbenzene in said unit leads to a reduction in the concentration of ethylbenzene of the adsorption feedstock,
the productivity of the process for separation of ethylbenzene in the second adsorption unit is improved because of the absence of paraxylene in the feedstock of said unit,
the use of a catalyst that comprises an EUO-structural-type zeolite in the isomerization zone makes it possible to improve the performance levels and also to increase the yield per pass of paraxylene, i.e., the recycling rate is lower, the isomerization feedstock volume is lower and the catalyst volume that is used for the isomerization is reduced,
the absence of ethylbenzene in the isomerization feedstock makes it possible to operate the isomerization unit under much less rigorous conditions than conventional units (a temperature of less than 20 to 30xc2x0 C., low hydrogen pressure) and with better productivity (hourly volumetric flow rate greater than 20 to 50%). As a result, the production of undesirable by-products is avoided, unlike the converting and dealkylating isomerizations of the prior art,
finally, by producing at the outlet of the first adsorption unit two fractions (raffinates, for example), of which one that is very concentrated in ethylbenzene becomes the feedstock for the second adsorption unit, the size of said unit is reduced while its productivity is increased.
It is possible to distill the second R1 ethylbenzene-containing fraction to eliminate at least a portion of the first desorbent and to recover the ethylbenzene-containing fraction that is introduced into the second adsorption unit for producing the essentially pure ethylbenzene and said fourth fraction that contains essentially metaxylene and orthoxylene.
This fourth fraction can be distilled so as to eliminate at least a portion of the second desorbent before being isomerized under mild conditions.
According to another characteristic of the process, when the draw-off of said raffinate that contains essentially metaxylene and orthoxylene is initiated in the first adsorption unit of fraction R2, it is possible to distill this fraction so as to eliminate at least a portion of the first desorbent, before being isomerized. If it is necessary to do so, it may be advantageous to distill fourth fraction (13, 17) from which desorbent is removed so as to recover an essentially pure metaxylene distillate (19) and an essentially pure orthoxylene residue (20).
It becomes very advantageous to distill the first and second desorbents in the same distillation column when, quite clearly, the first and second desorbents are identical. This is particularly the case when the operation is carried out with toluene in the two adsorption units or with paradiethylbenzene.
It still remains possible to use a first desorbent in the first adsorption unit that is different from the second desorbent in the second unit.
It is possible to distill fraction (R2) from which the first desorbent is removed so as to recover an essentially pure metaxylene distillate and an essentially pure orthoxylene residue. This distillation can advantageously be carried out in the same distillation column as the one that is used to treat the fourth orthoxylene and metaxylene fraction that is drawn off from the second adsorption unit.
According to a particularly advantageous characteristic, the first adsorption unit and the second adsorption unit use the principle of chromatography in a simulated moving bed, in simulated countercurrent according to U.S. Pat. No. 2,985,589 or with simulated co-current according to U.S. Pat. No. 4,498,991 and U.S. Pat. No. 4,402,832. More specifically, it may be advantageous to operate the first unit under the following conditions:
Simulated countercurrent:
Temperature: 100 to 200xc2x0 C.
Pressure: 2 to 30 bars (1 bar=105 Pa)
Desorbent to hydrocarbon-containing feedstock ratio: 1 to 2
Number of beds: 6 to 24
Desorbent: toluene or paradiethylbenzene
The adsorbents can be, for example, one of the those that are described in Patents U.S. Pat. No. 3,626,020 and U.S. Pat. No. 3,878,127.
More particularly, an X zeolite that is exchanged with barium and hydrated or a Y zeolite that is exchanged with potassium and barium is used. Toluene or para diethylbenzene will preferably be used as a desorbent.
Of course, the desorbent will be selected based on the adsorbent.
The second adsorption unit for separating ethylbenzene from the mixture that comprises ethylbenzene, metaxylene, and orthoxylene can be operated under the following conditions:
Simulated countercurrent:
Number of beds: 6 to 24
Temperature: 100 to 200xc2x0 C.
Pressure: 2 to 30 bars
Desorbent: toluene or paradiethylbenzene
Desorbent to feedstock ratio: 1 to 3
The zeolitic adsorbent of the second adsorption unit that feeds ethylbenzene can contain at least one element that is selected from the group of elements K, Rb, Cs, Ba, Ca, and Sr and optionally water. The conditions of this particular adsorption are described in, for example, U.S. Pat. Nos. 5,453,560, 4,613,725, 4,108,915, 4,079,094 and 3,943,182.
Titanosilicate-containing adsorbents that preferably have a pore opening on the order of 8 xc3x85, for example, such as those that are described in U.S. Pat. Nos. 5,244,650, 5,011,591, and U.S. Pat. No. 4,853,202, if the ethylbenzene/metaxylene selectivity is high, allow excellent separation and can provide excellent results.